(366q) Supported Ionic Liquid Membranes for Spaceflight Gas Separations

In-situ Resource Utilization (ISRU) technologies are critical to enable both long-duration crewed missions and robotic sample-return missions to the outer solar system. Providing all requisite mission consumables at launch becomes costly or impossible for long-duration, remote missions. Each ton of fuel produced through in-situ generation on the lunar surface will save a mission at least 7.5 tons of original payload mass. Reducing or even eliminating Earth-launched supplies greatly reduces launch costs and advances operational flexibility via more robust resource utilization capabilities.

Environmental Control and Life Support Systems (ECLSS) ensure a habitable environment for crew members through the monitoring, provision, and removal of metabolic inputs and outputs. ECLSS technologies must be reliable, and effective for the entire mission duration. These criteria are even more critical in long-duration missions, where remote settings and mission mass constraints make repairing or replacing technologies challenging. Atmosphere revitalization, the process by which the internal atmosphere of a spacecraft cabin or long-term habitat is maintained at suitable conditions and compositions for human life, is a critical component of any crewed mission. A crew’s daily oxygen demand is estimated to be approximately 0.82 kg O₂/person-day. The resulting CO₂ output is estimated to be 1.08 kg CO₂/person-day. Atmospheric CO₂ control (and subsequent oxygen regeneration) enables a breathable environment for astronauts.

The maximum permissible limit for CO₂ concentration for the International Space Station (ISS), as determined by NASA to maintain crew health and performance, is 3 mmHg (0.4 kPa). An atmosphere revitalization system will need to remove CO₂ at these concentrations while rejecting other atmospheric constituents like nitrogen, oxygen, and trace contaminants. CO₂ removal technologies may also be used to support ISRU processes. For example, CO₂ removal during a Mars mission may enable CO₂ provision to a plant growth habitat, supply as feedstock for a rocket-fuel producing Sabatier reactor, or as feedstock for oxygen production.

Ionic liquid (IL) sorbents are attractive for spaceflight applications due to their low volatility and high tunability for long-duration, low-maintenance, and task-specific performance at reduced pressures. Typical ionic liquid-based gas separation systems involve contacting an ionic liquid flow with gas flows at two separate stages to enable continuous operation: one stage for selective absorption of the target gas species and one for desorption. An IL gas separation system for human spaceflight must be capable of rapid, repeatable uptake and release of the desired gas constituent while also considering weight, space, and power. One promising configuration is a supported ionic liquid membrane (SILM) – a porous membrane filled with an ionic liquid sorbent. SILMs allow for continuous in-line separation and operate without a liquid flow. Instead, there are two gas flows – the feed gas mixture flows over one side of the membrane, where it contacts the supported liquid sorbent, and the liquid then releases the captured target molecule into the gas phase on the other side of the membrane. This process runs at steady state; components can be sized for operation at their maximum efficiencies, therefore reducing power requirements and increasing fidelity through the extension of component lifespans and the elimination of regularly cycling valves. A vacuum pump or low-pressure sweep gas on the permeate side forms the partial pressure-swing across the membrane.

In this work, we present novel SILM configurations and their performance in ECLSS- and ISRU-relevant separation processes. We manufactured SILMs using the ionic liquids propylammonium nitrate, 1-butyl-3-methylimidazolium acetate, and 1-ethyl-3-methylimidazolium acetate using different polypropylene and nylon membrane supports in both flat sheet and hollow fiber configurations and explore the CO₂ separation and mass transfer performance these SILMs. Gas transport experiments were conducted at feed CO₂ concentrations between 400 ppm and 100% to replicate spacecraft habitat conditions as well Martian atmosphere ISRU applications, with the balance composed of inerts or air, depending on the application. We explore the effects of IL, membrane material, pore size, temperature, humidity, and CO₂ concentration on membrane stability, total mass transport, CO₂ permeance, and selectivity of CO₂ over O₂ (for ECLSS cases) and CO (for ISRU cases). Polypropylene SILMs outperform nylon SILMs in CO₂ permeance but express instability via dewetting of the sorbent from the membrane pores. In tests with a 5% CO₂/95% air process gas, CO₂ permeance ranged from 7.7x10-7 scc/cm²-s-cmHg at 36°C to 24x10-6 scc/cm²-s-cmHg at 100°C for a nylon membrane wetted with 1-butyl-3-methylimidazolium acetate. The custom hollow fiber SILM yielded a CO₂ permeance of 7.49 x 10-6 scc/cm²-s-cmHg with a CO₂:O₂ selectivity of 49:1 at 20°C.

This work has furthered understanding of SILM functionality and showcases the promising potential of SILM-facilitated gas separation systems in spaceflight applications. It indicates that further work should include: techniques to modify wetting behavior and increase bubble-point, exploration of contaminant effects on permeance and stability, long-term shelf life, and exploration of system effects in different space mission architectures – namely mass, power, and volume requirements.

Though the SILMs reported here display a much lower mass transfer coefficient and lower CO2 flux than the contactors that utilize a liquid flow, the simplicity of SILMs could allow them to be more robust and demand less power and mass than liquid-flow systems for long-duration missions.

Note:

The pore size and humidity studies are currently ongoing, so results are not yet available. However, they will be presented at the conference and, if allowed, the abstract will be revised to include a summary of the results and their implications.

Research Interests

ionic liquids, membrane gas separations; deployable multilayer insulation for spacecraft; heterogeneous catalysis for organic reactions; chemical reaction engineering

What is the self-identified level of presentation experience for the potential presenter?

Thus far, I have presented 4 conference papers and 3 posters during my PhD, as well as a research thesis and 2 posters during my undergrad.